The present invention relates to a digital modulation signal measuring device and method, and more particularly to a digital modulation signal measuring device and method using a technique for evaluating the spectrum of a noise component included in a digital modulation signal, in a digital modulation signal measuring device for measuring a digital modulation signal.
Recently, a digital modulation signal is frequently used in mobile communication, broadcast, cable network communication, and the like.
This digital modulation signal is generated by quadrature-modulating a carrier signal by a modulation signal and transferred to a communication network.
When evaluating the quality of an apparatus and a channel using such a digital modulation signal, generally, code error rate is measured by a error measuring device, and transfer bandwidth or leakage of adjacent channels is measured by a spectrum analyzer.
For example, in a transfer channel of a bidirectional CATV network or the like, there is a problem of deteriorating the quality of the transfer channel caused by a noise generated by a household electrical appliance or leakage of a broadcast signal.
A periodical continuous noise such as CW causes more deterioration owing to code error even at the same level ratio, than a white noise having no self-correlation.
In case of laying a network, it is necessary to recognize the quality of the noise accurately and provide a stable service eliminating its cause.
Since a method of modulating the amplitude and the phase essentially is used for a digital modulation signal, it is often difficult to accurately recognize the quality of the noise component included in the digital modulation signal accurately.
When this digital modulation signal is measured by a spectrum analyzer, the occupation frequency band is all covered with signal spectrums Sp, as illustrated in FIG. 14.
Even if detecting an error by the error measurement, it is impossible to accurately recognize the quality of the noise included in the digital modulation signal and it is difficult to specify its cause.
In order to solve this problem, it may be possible to measure only the spectrum of a noise by a spectrum analyzer, using a single wave or in a non-signal state.
The above, however, cannot recognize the noise component of IM (intermodulation) which is generated in a relay or a receiver in a channel, caused by transfer of a digital modulation signal.
An object of the present invention is, in order to solve the above problem, to provide a digital modulation signal measuring device and method capable of easily recognizing the quality of a noise component included in a digital modulation signal.
One aspect of the present invention provides a digital modulation signal measuring device comprising:
a base band component extracting unit (21) for extracting a base band component included in a digital modulation signal to be measured and outputting the same as a digital signal string (binary sequence);
a frequency deviation correcting unit (30) for detecting carrier frequency deviation of the signal string of the base band component extracted by the base band component extracting unit and correcting the frequency deviation of the signal string of the base band component by the detected carrier frequency deviation;
an ideal signal string generating unit (40) for generating a signal string of an ideal base band component from the signal string of the base band component corrected by the frequency deviation correcting unit;
a noise signal detecting unit (45) for detecting a signal string of a noise component included in a digital modulation signal to be measured, by calculating difference between the signal string of the base band component corrected by the frequency deviation correcting unit and the signal string of the ideal base band component generated by the ideal signal string generating unit;
a spectrum calculating unit (50) for calculating spectrums of the base band component and the noise component, by performing discrete Fourier transform on the signal string of the base band component corrected by the frequency deviation correcting unit and the signal string of the noise component detected by the noise signal detecting unit; and
output units (55, 60) for outputting at least one of the spectrums of the base band component and the noise component calculated by the spectrum calculating unit.
Another aspect of the present invention provides a digital modulation signal measuring method comprising the steps of:
extracting a base band component included in a digital modulation signal to be measure and outputting the same as a digital signal string;
detecting carrier frequency deviation of a signal string of the extracted base band component and correcting the detected frequency deviation of the signal string of the base band component according to the detected carrier frequency deviation;
generating a signal string of an ideal base band component from the signal string of the corrected base band component;
detecting a signal string of a noise component included in the digital modulation signal to be measured, by calculating difference between the signal string of the corrected base band component and the signal string of the generated ideal base band component;
calculating spectrums of the base band component and the noise component, by performing discrete Fourier transform on the signal string of the corrected base band component and the signal string of the detected noise component; and
outputting at least one of the spectrums of the base band component and the noise component calculated in the above.